const correctness (espressif#89)

make IPAddress.toString() const method, it doesn't modify anything within the class.

Author: Palatis

cores/esp32/IPAddress.cpp

@@ -72,7 +72,7 @@ size_t IPAddress::printTo(Print& p) const
  return n;
 }
 
-String IPAddress::toString()
+String IPAddress::toString() const
 {
  char szRet[16];
  sprintf(szRet,"%u.%u.%u.%u", _address.bytes[0], _address.bytes[1], _address.bytes[2], _address.bytes[3]);
@@ -119,4 +119,4 @@ bool IPAddress::fromString(const char *address)
  }
  _address.bytes[3] = acc;
  return true;
-}
+}

cores/esp32/IPAddress.h

@@ -81,7 +81,7 @@ class IPAddress: public Printable
  IPAddress& operator=(uint32_t address);
 
  virtual size_t printTo(Print& p) const;
- String toString();
+ String toString() const;
 
  friend class EthernetClass;
  friend class UDP;